This invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves having the CHA framework type. In particular the present invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves having the CHA framework type using templates that contain at least one dimethylamino moiety in combination with a source of fluoride.
Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO""s). There are a wide variety of SAPO molecular sieves known in the art, of these the more important examples include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO41, and SAPO-56. For the methanol-to-olefins process SAPO molecular sieves having the CHA framework and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 xc3x85, and cylindrical cages within the structure of approximately 10xc3x976.7 xc3x85 type (xe2x80x9cAtlas of Zeolite Framework Typesxe2x80x9d, 2001, 5th Edition, p. 96). Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of AlPO4 and SAPO molecular sieves is a complicated process. There are a number of variables, which need to be controlled in order to optimise the synthesis in terms of the purity, yield, and quality of the molecular sieve produced. Of these variables the choice of template (hereinafter also referred to as templating agent) is usually one of the most important in determining which framework type is obtained.
One desirable group of silicoaluminophosphate molecular sieves are those that have low silicon contents. Silicoaluminophosphates of the CHA framework type with low silicon contents are particularly desirable for use in the methanol-to-olefins process. Wilson, et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (Microporous and Mesoporous Materials, 29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
In U.S. Pat. No. 4,440,871 (Lok et.al) the synthesis of a wide variety of SAPO materials of various framework types are described with a number of specific examples. Also disclosed are a large number of possible organic templates, with some specific examples. In the specific examples a number of CHA framework type materials are described. The preparation of SAPO-34 is reported, using tetraethylammonium hydroxide (TEAOH), or isopropylamine, or mixtures of TEAOH and dipropylamine (DPA) as templates. Also disclosed in this patent is a specific example that utilises cyclohexylamine in the preparation of SAPO-44. Although other template materials are described in this patent there are no other templates indicated as being suitable for preparing SAPO""s of CHA framework type. Certain aminoalcohols are mentioned, including triethanolamine, N-methyldiethanolamine, N-methylethanolamine, N,N-dimethylethanolamine and N,N-diethylethanolamine as possible templates for SAPO molecular sieves. Of these materials N,N-diethylethanolamine is shown to produce SAPO-5, which is of framework type AFI. For the other aminoalcohols no indication is provided as to which SAPO or which framework type may be obtained through their use.
Since the synthesis of SAPO-34 was reported in U.S. Pat. No. 4,440,871, tetraethylammonium hydroxide (TEAOH) either alone, or in combination with dipropylamine (DPA), has been the preferred template for preparing SAPO-34. However, there are problems associated with the use of TEAOH and DPA. When used alone, TEAOH affords a limited range of synthesis parameters. For example, under certain conditions TEAOH will also template the synthesis of SAPO-18 which has the AEI framework type. TEAOH is thus relatively intolerant to synthesis condition variations. TEAOH is sometimes combined with DPA. However, DPA has a low boiling point (110xc2x0 C.) resulting in the need for production plants that can handle high pressures. In certain countries, the use of DPA requires special regulatory authorizations due to its toxicity. Also, DPA is an aggressive template and is often implicated in re-dissolution of the silicoaluminophosphate molecular sieve during its synthesis, resulting in poor quality crystalline product due to surface pitting of the crystals. Finally, it has proved difficult up to now to make pure phase CHA silicoaluminophosphate molecular sieves with a low silica to alumina ratio.
In U.S. Pat. No. 4,440,871, it was reported that SAPO-44 was obtained xe2x80x9cas the major phasexe2x80x9d using cyclohexylamine as a template. In U.S. Pat. No. 6,162,415 (Liu, et.al.), relatively pure CHA SAPO-44 was obtained using the same template but with control of the ratio of template to aluminium source and the ratio of phosphorous source to aluminiumn source.
In European Patent Publication No. 0,993,867, it was reported that the use of methylbutylamine resulted in SAPO-47 and the use of cyclohexylamine resulted in impure SAPO-44. Methylbutylamine has an even lower boiling point, at 91xc2x0 C., than DPA.
U.S. Pat. No. 4,861,739 (Pellet, et al.), Example 102, it was reported that the use of N,N-diethylethanolamine produced CoAPSO-47, having Si concentrated on the peripheries of the crystal and Co at the centre.
In U.S. Pat. No. 4,310,440 (Wilson et.al), triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine, were all used to prepare AlPO4-5, aluminophosphates of framework type AFI. N-methylethanolamine was also reported to produce AlPO4-21 of framework type AWO.
In European Patent Publication No. 0,993,867, it was reported that diethanolamine produced SAPO-34 and SAPO-5 under different synthesis conditions.
In the art various attempts have been made to improve the synthesis of AlPO4 or SAPO molecular sieves. One approach has been the addition of a source of fluoride ions to the synthesis mixture.
In U.S. Pat. No. 5,096,684 (Guth et.al.), morpholine and tetraethylammonium hydroxide were found to template the production of SAPO-34 when in the presence of HF. According to Guth et.al. the use of HF in combination with the organic template results in silicoaluminophosphates which have improved thermal and hydrolytic stability.
In U.S. Pat. No. 4,786,487 (Kuehl et.al.), SAPO-20 was produced from synthesis mixtures containing tetramethylammonium hydroxide and fluoride ions from water soluble sources of fluoride such as Na, K and ammonium fluoride.
In U.S. Pat. No. 6,001,328 (Lillerud et.al.), a silicoaluminophosphate indicated as UiO-S7 was prepared using tetramethylammonium hydroxide pentahydrate or tetramethylammonium hydroxide, in combination with HF.
In a Ph.D. thesis (E. H. Halvorsen, University of Oslo, 1996), it was reported that low silica SAPO-34, designated as UiO-S4, was produced using TEAOH template in combination with HF.
Wilson et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (Microporous and Mesoporous Materials, 29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
As can be seen from the disclosures described herein, there have been a number of attempts to find alternative templates for the CHA framework type with limited success. It is desirable therefore to find new templates and template systems that are specific for the synthesis of molecular sieves having the CHA framework type. In addition there is a need for new template systems that afford a more effective control of the final composition of the SAPO molecular sieve materials and, in particular, of the Si content of the final product. In addition a further need is to obtain SAPO materials having the CHA framework type that have a low acid density, which is directly related to low silica content.
The present invention provides a method for preparing aluminophosphate or silicoaluminophosphate molecular sieves of CHA framework type, the process comprising the steps of;
a) forming a reaction mixture comprising a source of aluminum, a source of phosphorus, at least one source of fluoride ions and at least one template containing one or more dimethylamino moieties, and optionally a source of silicon,
b) inducing crystallization of aluminophosphate and/or silicoaluminophosphate molecular sieve from the reaction mixture,
c) recovering aluminophosphate and/or silicoaluminophosphate molecular sieve from the reaction mixture.
The process of the present invention results in new molecular sieve materials, which, as synthesized, have unique X-ray diffraction patterns. Thus the process of the present invention provides for a variety of new materials that have the XRD patterns as described below.
In one embodiment of the present invention there is provided an aluminophosphate or silicoaluminophosphate molecular sieve, substantially of CHA framework type, comprising, preferably within its intracrystalline structure, fluoride and at least one template which contains one or more N,N-dimethylamino moieties.
In another embodiment of the present invention there is provided a molecular sieve, which has a characteristic X-ray powder diffraction pattern containing at least the d-spacings as set forth in Table Ia:
In another embodiment of the present invention there is provided a molecular sieve, which has a characteristic X-ray powder diffraction pattern containing at least the d-spacings as set forth in Table Ib:
It is preferred that the molecular sieve as identified in Table Ia or Ib comprises N,N-dimethylethanolamine and fluoride, more preferably within its intra-crystalline structure.
In yet another embodiment of the present invention there is provided a molecular sieve, which has a characteristic X-ray powder diffraction pattern containing at least the d-spacings as set forth in Table IIa:
In a further embodiment of the present invention there is provided a molecular sieve, which has a characteristic X-ray powder diffraction pattern containing at least the d-spacings as set forth in Table IIb:
It is preferred that the molecular sieve as identified in Tables IIa or IIb comprises N,N-dimethylpropylenediamine and fluoride, more preferably within its intra-crystalline structure.
In another embodiment the present invention provides a method for the manufacture of a molecular sieve catalyst composition, the method comprising forming a molecular sieve catalyst composition from a mixture comprising at least one formulating agent and at least one aluminophosphate and/or silicoaluminophosphate molecular sieve of CHA framework type comprising, preferably within its intra-crystalline structure, fluoride and at least one template containing one or more N,N-dimethylamino moieties, or at least one aluminophosphate or silicoaluminophosphate molecular sieve as obtained from a process utilising a template comprising one or more N,N-dimethylamino moieties and a source of fluoride ions.
In yet a further embodiment the present invention provides for a molecular sieve catalyst composition comprising at least one aluminophosphate and/or silicoaluminophosphate molecular sieve of CHA framework type comprising, preferably within its intra-crystalline structure, fluoride and at least one template containing one or more N,N-dimethylamino moieties, or at least one aluminophosphate or silicoaluminophosphate molecular sieve as obtained from a process utilising a template comprising one or more N,N-dimethylamino moieties and a source of fluoride ions, in admixture with at least one formulating agent.
In a further embodiment the present invention provides for the use of a template comprising one or more N,N-dimethylamino moieties in combination with a source of fluoride ions in the synthesis of aluminophosphate or silicoaluminophosphates molecular sieves of CHA framework type.